Diffusion cooled, radio frequency (RF) pulsed CO2 gas discharge lasers driven by a radio frequency power supply (REPS) are used extensively for laser processing applications such as cutting, drilling, scribing, fusing, marking, heat treating, and engraving. For many of these applications, good pulse-to-pulse amplitude stability of the output power of the lasers is required. Power output stability from a cold start of better than 3% is a typical specification while a 1% requirement is becoming more common.
While power control methods have been developed to achieve such specifications during a steady state operation, it has been found that there is a “warm-up” period following a cold start after a long period of inactivity, prior to reaching steady-state operation, where the laser-output power rises significantly above the controlled steady-state level. There is also a measurable power rise after a relatively short period of inactivity. This reduces the usefulness of the laser during that warm-up period.
One example of a power rise during warm up is depicted graphically in FIG. 1. In this example, a pulsed 346 Watt (W) output power RF excited slab CO2 laser was operated at a pulse repetition frequency (PRF) of 5 kilohertz (kHz) with pulses of 30 microseconds (μs) duration. In FIG. 1 the measured output power as a function of time is schematically depicted after the laser had been restarted after being turned off for a period of only 30 seconds. The laser used a temperature controlled closed cycle chiller to minimize the laser's steady state output power variations with temperature. The laser power was 376 W approximately 0.5 minutes after turn on. The power dropped to 350 W after approximately 6 minutes and then gradually dropped down to 346 W steady state approximately 16 minutes after turn-on. The percentage of output power variation was approximately 7.5% 6 minutes after turn or, and 8.7% 16 minutes after turn on. As the laser turn off time was increased, the percentage drop in the output power after restart increased.
It was determined by calculation and experiment that the power-rise during warm up could be attributed to a lower-than-steady-state temperature of the discharge gas at the instant of turn on of the laser after even a relatively short period of inactivity. The output power of CO2 diffusion cooled lasers decreases as their gas temperature increases. It was determined that prior to being turned on after being off for seconds or longer, the gas temperature was low enough to account for the sharp rise of output power during warm-up to the steady state operating temperature. It was calculated that the power output drops by about 0.6% per degree C. with increasing temperature. As the gas heats up after turn on, the output power of the laser drops and eventually reaches a steady state value.
A diffusion-cooled CO2 laser is a gas discharge laser in which cooling of the discharge occurs by having a relatively small separation between parallel cooled electrodes between which the discharge is formed when RF power is applied to the electrodes. For lasers having an output larger than about 70 W, the electrodes are usually cooled by flowing a liquid coolant through cooling lines in contact with the electrodes, or through drilled passages within the electrodes.
The separation between the electrodes is intentionally sufficiently small in order to provide a high probability that excited state CO2 molecules residing in a relatively long-lifetime “010” bending vibration state (a non-lasing state that is located only marginally above the ground state), can collide with the liquid cooled electrode surfaces. This collision process depopulates the “010” state, thereby cooling the gas during discharge. The depopulation of the “010” lower level increases the population inversion between two “lasing” levels. This results in higher laser output power and efficiency.
Providing effective cooling of the electrodes and, accordingly, the laser discharge, is important in order to obtain good laser output efficiency from diffusion cooled, hermetically sealed, RF excited CO2 lasers. For demanding laser material processing applications, temperature controlled closed cycle chillers are used to minimize steady state output power variations with discharge temperature. It is believed that this cooling is sufficiently effective that the lasing gas temperature can drop significantly even after only a relatively short period such as the 30 seconds in the example of FIG. 1. It is believed also that this effectiveness of the cooling is also responsible for the relatively long period required for warm-up to steady state operation.
Continued discussion of this assumed mechanism for high power during warm-up is set forth below with reference to the graphs of FIGS. 2A, 2B, and 2C. Before discussing the graphs in detail, however, it is useful to briefly discuss the mechanism by which RF electrodes are typically driven.
Typically the electrodes are driven by an RFPS that is powered by a DC power supply. In simple terms, the RF power supply “converts” DC voltage supplied by the DC power supply to a corresponding RF voltage, and the converted RF voltage is amplified in the RFPS to a level sufficient to drive the electrodes with a power sufficient to provide a desired laser output power. Laser manufacturers usually make use of commercially produced DC power supplies to gain lower cost from the increased volume whereas they usually design and build (or have built) RF power supplies appropriate for a particular laser family.
The design of modern, diffusion-cooled RF-excited CO2 lasers is such that once steady-state output power has been established there will be little unintended variation (fluctuation) of that output power. This is fortunate as closed loop RF power control arrangements for stabilizing output are either impractical or expensive. A desired increase or decrease in steady state output is usually provided by pulse-width modulation means. This involves respectively increasing or decreasing output pulse duration while maintaining a fixed PRF for the output pulses.
FIG. 2A is a graph of RF power applied to electrodes as a function of time. The RF power (PHL) represented can be either continuous or in the form of a continuous pulse train having a PRF anywhere from about 100 (Hertz) Hz up to 200 kHz. This makes no difference to the analysis. At time t0, the power is turned on after an assumed long period of inactivity. At an arbitrary time t2, the RF power is turned off. At a time t3 the RF power is turned back on again. The inactivity period (t3-t2), here, is assumed to be shorter than the inactivity period preceding time t0.
FIG. 2B schematically illustrates discharge (lasing gas) temperature as a function of time corresponding to the RF power application as a function of time depicted in FIG. 2A. At time t0, the temperature TC of the lasing gas the discharge electrodes is equal to the temperature of the electrodes, typically about 23° C. for cooled electrodes. As soon as a discharge is lit, the lasing gas temperature begins to rise. The lasing gas temperature reaches a steady state temperature, TH at time t1. When the RF power is turned off at time t2, the lasing gas temperature begins to fall, reaching a temperature TC′ at time t3. Here it is assumed that the turn-off period (t2-t3) is sufficiently shorter than the original turn-off period (before t0) that TC′ is greater than TC. At time t3 the RF power is turned back and the gas temperature begins to increase again. At time t4, the gas temperature again reaches steady state temperature TH.
FIG. 2C is a graph schematically illustrating laser output power as a function of time under the RF power and gas-temperature conditions of FIGS. 2A and 2B. At time t0, the gas is the coolest and the laser output power accordingly is the highest with a value designated as PC. As the gas heats up, the laser power drops until a steady state value PH is reached at time t1. At time t2, the RF power is turned off by a user's command signal which quickly turns off the laser output power. The RF power is again turned on at t3, At his time the discharge temperature has only fallen to a vale TC′ (see FIG. 2B) which is higher than the value TC at t0. Because of this, the laser power rises only to a value PC′, which is not as high as PC. At time t4, the laser power again reaches steady state PH.
For demanding laser material processing applications, such output power variation after restarting is inconvenient. Accordingly, there is a need to mitigate if not altogether eliminate the power rise during the warm-up period.